Shaft shear detection through shaft oscillation

ABSTRACT

There is described a shaft shear event detection method. The method comprises storing in memory a shaft oscillation signature determined as a function of known characteristics of the shaft and associated with a shaft shear event; monitoring a rotational speed of the shaft; detecting from the rotational speed an oscillation wave superimposed on the rotational speed by detecting a first period below a lower threshold and a second period above an upper threshold, and detecting a rate of occurrence of the first period and the second period, the oscillation wave having a wave modulation frequency corresponding to the rate of occurrence and a wave modulation amplitude; comparing the oscillation signature to the oscillation wave; and detecting the shaft shear event when the oscillation wave corresponds to the oscillation signature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.14/997,913 filed on Jan. 18, 2016, the contents of which are herebyincorporated by reference in their entirety.

TECHNICAL FIELD

The application relates generally to detecting shaft shears and, moreparticularly, to detecting shaft shears of loaded, rotating shafts,positioned between a source and a load.

BACKGROUND OF THE ART

The low pressure shaft on a gas turbine engine connects the lowerpressure turbine to the fan, and transfers the power from the turbine tothe fan. The transferred power is then converted into engine thrust.During engine operation, the shaft experiences very high torsionalloads. In the unlikely event of a shaft shear and loss of load, the fuelmust be shut off quickly to prevent damage to the engine.

Several methods exist for detecting shaft shear. For example, mechanicalaxial detection involves using a probe or sensor adjacent to a rear endof the shaft to detect the axial motion of the shaft after the shear.The sheared shaft collides with the sensor, resulting in a fuel shutoff.Another example comprises using a processor to calculate a rate ofchange of shaft speed. When the rate of change falls below a certainvalue for a period of time, a fuel shutoff is commanded.

Methods that involve delayed indicators, such as axial displacement andaxial movement, or time-consuming signal processing, such as rate ofchange of shaft speed, are not well-suited for a process requiring rapidfuel shutoff. In addition, methods that require special sensors oradditional hardware also have certain disadvantages, such as additionalcost and weight, and/or give rise to durability and reliabilityconcerns.

There is therefore a need to improve on existing methods for detectingshaft shear.

SUMMARY

In one aspect, there is provided method for detecting a shear of arotating shaft positioned between a source and a load. The methodcomprises storing in memory a shaft oscillation signature determined asa function of known characteristics of the shaft and associated with ashaft shear event; monitoring a rotational speed of the shaft; detectingfrom the rotational speed an oscillation wave superimposed on therotational speed by detecting a first period below a lower threshold anda second period above an upper threshold, and detecting a rate ofoccurrence of the first period and the second period, the oscillationwave having a wave modulation frequency corresponding to the rate ofoccurrence and a wave modulation amplitude; comparing the oscillationsignature to the oscillation wave; and detecting the shaft shear eventwhen the oscillation wave corresponds to the oscillation signature.

In another aspect, there is provided system for detecting a shear of arotating shaft positioned between a source and a load. The systemcomprises a memory storing a shaft oscillation signature determined as afunction of known characteristics of the shaft and associated with ashaft shear event; and at least one of at least one processor configuredfor executing program code and a circuit. The at least one of at leastone processor configured for executing program code and a circuit isconfigured for monitoring a rotational speed of the shaft; detectingfrom the rotational speed an oscillation wave superimposed on therotational speed by detecting a first period below a lower threshold anda second period above an upper threshold, and detecting a rate ofoccurrence of the first period and the second period, the oscillationwave having a wave modulation frequency corresponding to the rate ofoccurrence and a wave modulation amplitude; comparing the oscillationsignature to the oscillation wave; and detecting the shaft shear eventwhen the oscillation wave corresponds to the oscillation signature.

In a further aspect, there is provided a system for detecting a shear ofa rotating shaft positioned between a source and a load. The systemcomprises a memory storing a shaft oscillation signature determined as afunction of known characteristics of the shaft and associated with ashaft shear event; a speed sensing device for monitoring a rotationalspeed of the shaft; at least one processor configured for executingprogram code for detecting from the rotational speed an oscillation wavesuperimposed on the rotational speed by detecting a first period below alower threshold and a second period above an upper threshold, anddetecting a rate of occurrence of the first period and the secondperiod, the oscillation wave having a wave modulation frequencycorresponding to the rate of occurrence and a wave modulation amplitude;comparing the oscillation signature to the oscillation wave; anddetecting the shaft shear event when the oscillation wave corresponds tothe oscillation signature.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures in which:

FIG. 1 is a schematic cross-sectional view of a gas turbine engine;

FIG. 2 is an illustration of a frequency modulation signal composition;

FIG. 3 is an example embodiment of a speed sensing device;

FIG. 4 is an example embodiment of a raw speed signal resulting from ashaft shear;

FIG. 5 is a flowchart of a shear shaft detection method, in accordancewith an embodiment;

FIG. 6 is a block diagram of an embodiment of the detection methodimplemented in a hardware circuit;

FIG. 7 is a block diagram of an embodiment of the detection methodimplemented in software and hardware; and

FIG. 8 is a block diagram of an embodiment of an application running onthe processor of FIG. 7.

DETAILED DESCRIPTION

FIG. 1 illustrates a gas turbine engine 10 of a type that may beprovided for use in subsonic flight, generally comprising in serial flowcommunication a fan 12 through which ambient air is propelled, acompressor section 14 for pressurizing the air, a combustor 16 in whichthe compressed air is mixed with fuel and ignited for generating anannular stream of hot combustion gases, and a turbine section 18 forextracting energy from the combustion gases. A shaft 20 is providedbetween the turbine 18, and the fan 12. A shaft shear 22 may occur atany point along the shaft 20. When the shaft 20 shears, the suddenunloading results in a rapid untwisting of the shaft 20. The untwistingcauses a unique oscillation or ringing of the shaft 20. This ringing maybe identified as a unique oscillation signature for detecting the shaftshear 22 and triggering a fuel shutoff command. Note that while theturbine engine 10 illustrated in FIG. 1 is a turbofan engine, thedetection methods and systems described herein may also be applicable toturboprop engines and turboshaft engines. In addition, the teachingsherein are not limited to turbine engines as a shear of any rotatingloaded shaft provided between a source (such as a turbine) and a load(such as a fan) may be detected using the unique oscillation signaturethat results from a shaft shear event.

The shaft shear 22 manifests itself as an oscillation wave that issuperimposed on the shaft speed signal and may be used as a signatureindicative of a shear for a given shaft. The superimposed oscillationwave will be referred to herein as a shaft oscillation signature. Thewaveform that results from the oscillation wave being superimposed onthe shaft speed signal, referred to herein as a resultant modulatedwaveform, is composed of a carrier wave and a modulating wave. Thecarrier wave represents the speed of the shaft before the shear, and themodulating wave represents the oscillation wave due to the suddenunloading of the shaft 20, and thus the shaft oscillation signature.Referring to FIG. 2, there is illustrated an example of a carrier wave202, a modulating wave 204, and a resultant modulated waveform 206obtained from the combination of the carrier wave 202 and the modulatingwave 204. All three waves 202, 204, 206 are plotted as displacement (yaxis) versus time (x axis). The carrier wave 202 is the speed of theshaft 20 before the shaft shear event. The modulating wave 204 is theoscillation of the shaft 20 immediately after the shaft shear event. Itis this oscillation that is detected as a shaft oscillation signaturefrom the resultant modulated waveform 206 in order to detect a shaftshear event.

The level of deviation of the modulation wave 204 from the carrier wave202 is a function of the torque experienced by the shaft 20 immediatelybefore the shear and corresponds to the amplitude of the modulation wave204, referred to herein as a wave modulation amplitude 208. Thefrequency of the modulation wave 204, referred to herein as a wavemodulation frequency, corresponds to the reciprocal of the period 210.The wave modulation frequency is independent of the torque on the shaft20 and is a function of the physical location of the shear on the shaftas well as the dynamics and physical characteristics of the rotatingsystem. Therefore, the unique shaft oscillation signature that resultsfrom a shaft shear event may be composed of a range of possible wavemodulation frequencies and wave modulation amplitudes, as a function ofpossible positions of shaft shear and possible torque levels applied tothe shaft 20, respectively. The applicable ranges of wave modulationfrequencies and wave modulation amplitudes may be determined beforehandusing various modeling techniques, known to those skilled in the art. Insome embodiments, the applicable wave modulation amplitudes may bedetermined, or narrowed from a broader range, during engine operation bymeasuring shaft torque or by monitoring other engine parameters fromwhich torque may be calculated.

Some examples of physical characteristics affecting the dynamics of therotating system include the shape of the shaft (including circumferenceand length), material properties of the shaft, damping characteristicsof the rotor system, and the characteristics of the speed sensing deviceused to obtain the rotational speed of the shaft. Various speed sensingdevices may be used to measure the rotational speed of the shaft. Thespeed sensing device should be selected such that the maximum possibleoscillation frequency resulting from a shear may be detected whileoperating at the lowest possible shaft speed. The speed sensing devicemay be contact-based or non-contact based. In some embodiments, acontact-based device may be composed of a phonic wheel assembly, asillustrated in FIG. 3. A phonic wheel 304 having a plurality of teeth306 distributed around an outer surface thereof may be placed in contactwith the rotating shaft 20. A sensor 302 is positioned relative to thephonic wheel 304. The rotating shaft 20 propels the phonic wheel 304,creating pulses that are read by the sensor 302 and converted intorevolutions per unit time. The sensor 302 may be a proximity sensor, anoptical sensor, an inductive sensor, or any other type of sensor knownto those skilled in the art. In some embodiments, the number of phonicwheel teeth 306 is selected such that the lowest speed signal frequencyis at least five times the highest possible oscillation frequencyresulting from the shear. In some embodiments, the number of phonicwheel teeth 306 is selected such that the lowest speed signal frequencyis at least six times the highest possible oscillation frequencyresulting from the shear. In some embodiments, the number of phonicwheel teeth 306 is selected such that the lowest speed signal frequencyis at least seven times the highest possible oscillation frequencyresulting from the shear. In some embodiments, the number of phonicwheel teeth 306 is selected such that the lowest speed signal frequencyis at least eight times the highest possible oscillation frequencyresulting from the shear. Other embodiments for the number of teeth 306of the phonic wheel may also be used in order to ensure that the phonicwheel 304 have enough teeth 306 to sample the highest possibleoscillation frequency at the lower possible speed.

In some embodiments, a non-contact device may be composed of a singleelectronic device, such as a Fast Synchronization Sensor (FSS), whichcomprises a magnetic dipole keyed to one end of the shaft by means of anon-magnetic holder, in front of which a magnetic encoder sensor ispositioned. In other embodiments, a non-contact device comprises a lightsource, such as a laser or infrared light, that is aimed at the rotatingshaft 20 to which one or more pieces of reflective tape have beenaffixed. As the light source hits the shaft 20, it is reflected off thetape and back to the sensor that converts the reflected lightmeasurements into revolutions per unit time. Other embodiments forsensing the rotational speed of the shaft 20 may also be used.

Turning now to FIG. 4, there is illustrated the carrier wave 402 (orshaft speed) and the resultant modulated waveform 404 before and after ashaft shear event 400. Before the shaft shear event 400, the carrierwave 402 is not modulated by any other waveform and therefore, isidentical to the resultant modulated waveform 404. After the shaft shearevent 400, an oscillation wave is superimposed on the shaft speed andcan be seen as the modulation wave 406. The carrier wave 402progressively slows as the shaft speed decreases. The resultantmodulated waveform 404 is modified by the modulation wave 406. Asillustrated, the modulation wave 406 will affect the period, and thusthe frequency, of the resultant modulated waveform 404. The instances ofhighest frequency 412 of the resultant modulated waveform 404 willcorrespond to the amplitude peaks 408 of the modulating wave 406, whilethe instances of lowest frequency 414 of the resultant modulatedwaveform 404 will correspond to the amplitude valleys 410 of themodulating wave 406.

Referring to FIG. 5, there is illustrated an embodiment of a method fordetecting a shear of a rotating shaft provided between a source and aload. At 502, one or more shaft oscillation signatures are stored inmemory. There may be a single shaft oscillation signature stored, or aplurality of shaft oscillation signatures, each one of the pluralitycorresponding to a specific shaft. Each shaft oscillation signature maybe composed of a single modulation frequency, referred to herein as asignature modulation frequency, and a single modulation amplitude,referred to herein as a signature modulation amplitude. Each shaftoscillation signature may alternatively comprise a range of signaturemodulation frequencies and/or signature modulation amplitudes. At 504,the rotational speed of the shaft is monitored. At 506, an oscillationwave superimposed on the rotational speed is detected. At 508, theoscillation wave and the shaft oscillation signature are compared. Whena match is determined, a shaft shear event is detected, as per 510. Ifthere is no match, the method continues to monitor the rotational speedof the shaft, as per 504.

In some embodiments, comparing the oscillation wave and the shaftoscillation signature, as per 508, comprises comparing a signaturemodulation frequency to a wave modulation frequency and comparing asignature modulation amplitude to a wave modulation amplitude. If theshaft oscillation signature comprises a range of signature modulationfrequencies and/or a range of signature modulation amplitudes, thencomparing the shaft oscillation wave to the shaft oscillation signaturecomprises determining if the wave modulation frequency and the wavemodulation amplitude fall within the range of signature modulationfrequencies and the range of signature modulation amplitudes,respectively.

In some embodiments, detecting from the rotational speed an oscillationwave, as per 506, comprises determining the wave modulation amplitudeand the wave modulation frequency directly from the resultant modulatedwaveform. For example, this can be done by decomposing the resultantmodulated waveform into a carrier wave and a modulation wave andextracting the amplitude and frequency from the modulation wave. Theresultant modulated waveform may be represented as follows:

${y(t)} = {A_{c}\mspace{14mu}{\cos\left( {{2\pi\; f_{c}t} + {\frac{f_{\Delta}}{f_{m}}{\cos\left( {2\pi\; f_{m}t} \right)}}} \right)}}$

where A_(c) is the carrier wave amplitude, f_(c) is the carrier wavefrequency, Δf is the wave modulation amplitude, and f_(m) is the wavemodulation frequency.

In some embodiments, detecting from the rotational speed an oscillationwave, as per 506, comprises determining the wave modulation amplitudeand wave modulation the frequency indirectly from the resultantmodulated waveform. For example, the method may involve detecting when aperiod of the resultant modulated waveform reaches below a lowerthreshold and above an upper threshold. Referring back to FIG. 4, thecarrier swing 416 is the total deviation of the waveform from a highestfrequency 412 to a lowest frequency 414. The highest frequency 412occurs when the period exceeds the upper threshold 418. The lowestfrequency 414 occurs when the period falls below the lower threshold420. The carrier swing corresponds to twice the wave modulationamplitude. The rate of occurrence of the period exceeding the upperthreshold 418 and falling below the lower threshold 420 isrepresentative of the wave modulation frequency.

The detection method described herein may be implemented in variousmanners, such as in software on a processor, on a programmable chip, oron an Application Specific Integrated Chip (ASIC), or as a hardwarecircuit. In some embodiments, the detection method is implemented inhardware on a dedicated circuit board located inside an ElectronicEngine Controller (EEC) or an Engine Control Unit (ECU). The EEC or ECUmay be provided as part of a Full Authority Digital Engine Control(FADEC) of an aircraft. In some cases, a processor may be used tocommunicate information to the circuit, such as shaft oscillationsignatures. In other embodiments, the detection method is implemented ina digital processor. In some embodiments, the FADEC performs theshutdown of the fuel once a shaft shear event has been detected.

An example embodiment of an implementation in hardware circuitry isillustrated in FIG. 6. The raw speed sensor frequency may be input to acircuit 600 comprising a frequency demodulator 602, for separating thecarrier wave from the modulation wave. The wave modulation frequency andwave modulation amplitude are provided to separate range comparators604, 606, respectively. If both the wave modulation frequency and thewave modulation amplitude are found to be within the ranges of thesignature modulation frequency and the signature modulation amplitude ofthe shaft oscillation signature, an AND gate 608 will send a signal to atimer or counter 612. When the timer or counter 612 has reached apredetermined time 610 for confirming a shaft shear event, a shaft shearis detected. The shaft shear detection signal may cause an alarm to ringor a warning message to be transmitted/displayed. The shaft sheardetection signal may also trigger a fuel shutoff command or may be usedas a fuel shutoff command. Different and/or additional components mayalso be used in the circuit 600 to perform the detection method asdescribed herein.

An example embodiment of software and hardware implementation isillustrated in FIG. 7. A system 700 for detecting a shaft shear eventmay comprise, amongst other things, one or more applications 706 ₁ . . .706 _(n) running on a processor 704 coupled to a memory 702. Processor704 may correspond to a plurality of processors. In addition, while theapplications 706 ₁ . . . 706 _(n) are illustrated and described asseparate entities, they may be combined or separated in a variety ofways. The memory 702 accessible by the processor 704 may receive andstore data. The memory 702 may be a main memory, such as a high speedRandom Access Memory (RAM), or an auxiliary storage unit, such as a harddisk, a floppy disk, or a magnetic tape drive, or any combinationthereof. The memory 702 may be any other type of memory, such as aRead-Only Memory (ROM), or optical storage media such as a videodisc anda compact disc. The processor 704 may access the memory 702 to retrievedata. The processor 704 may be any device that can perform operations ondata. Examples are a central processing unit (CPU), a front-endprocessor, a microprocessor, and a network processor. The applications706 ₁ . . . 706 _(n) are coupled to the processor 704 and configured toperform various tasks.

FIG. 8 illustrates an exemplary embodiment of application 706 ₁ runningon the processor 704. The application 706 ₁ illustratively comprises aspeed monitoring unit 802, an oscillation wave detection unit 804, and acomparison unit 806. The speed monitoring unit 802 may be configured tomonitor the rotational speed by receiving a raw speed signal from aspeed sensing device, as described above. The oscillation wave detectionunit 804 may be configured to detect an oscillation wave superimposed onthe rotational speed of the shaft, as per the embodiments describedherein. The comparison unit 806 may be configured to compare the shaftoscillation signature to the oscillation wave, as described herein. Oncea match is detected, a shaft shear detection signal and/or a fuelshutoff command may be output.

In some embodiments, the detection method is implemented using a phonicwheel sensing assembly for sensing the rotational speed. The toothpassing time of the phonic wheel may be stored in a buffer. A constanttooth passing time implies a constant speed. When the shaft is sheared,the tooth passing time oscillates with a certain deviation around thetooth passing time immediately before the shear. A processor or circuitmonitors each new tooth passing time and compares it with an average ofthe previous values. If the new passing time sometimes exceeds an upperbound limit and sometimes falls below a lower bound limit, the shaft isdetermined to be oscillating. A counter is incremented and if thecounter exceeds a certain value, shaft shear detection is confirmed.

In some embodiments, the detection method is combined with otherdetection methods in order to detect a larger spectrum of torque shaftshears. A torque below a minimum threshold may result in a wavemodulation amplitude that is indistinguishable from the speed signal.Therefore, a different detection method may be used for torque levelsbelow the minimum threshold. In some embodiments, the detection methodmay comprise monitoring a torque as applied to the shaft and performingthe method only when the torque meets the minimum threshold.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departing from the scope of the invention disclosed.For example, the detection method may be provided on non-transitorycomputer readable medium having stored thereon program code executableby a processor for performing the method. The blocks and/or operationsin the flowchart described herein are for purposes of example only.There may be many variations to these blocks and/or operations withoutdeparting from the teachings of the present disclosure. For instance,the blocks may be performed in a differing order, or blocks may beadded, deleted, or modified. Still other modifications which fall withinthe scope of the present invention will be apparent to those skilled inthe art, in light of a review of this disclosure, and such modificationsare intended to fall within the appended claims.

The invention claimed is:
 1. A method for detecting a shear of arotating shaft of a gas engine, the shaft positioned between a sourceand a load, the method comprising: storing in memory a shaft oscillationsignature determined as a function of known characteristics of the shaftand associated with a shaft shear event; monitoring a rotational speedof the shaft; detecting from the rotational speed an oscillation wavesuperimposed on the rotational speed by detecting a first period below alower threshold and a second period above an upper threshold, anddetecting a rate of occurrence of the first period and the secondperiod, the oscillation wave having a wave modulation frequencycorresponding to the rate of occurrence and a wave modulation amplitude;comparing the shaft oscillation signature to the oscillation wave;detecting the shaft shear event when the oscillation wave corresponds tothe shaft oscillation signature; and issuing a shaft shear detectionsignal in response to detecting the shaft shear event, the shaft sheardetection signal triggering a fuel shut-off of the gas engine.
 2. Themethod of claim 1, wherein the oscillation signature comprises asignature modulation frequency and a signature modulation amplitude, andwherein comparing the oscillation wave to the shaft oscillationsignature comprises comparing the signature modulation frequency to thewave modulation frequency and comparing the signature modulationamplitude to the wave modulation amplitude.
 3. The method of claim 2,wherein the oscillation signature comprises a range of signaturemodulation frequencies and a range of signature modulation amplitudes,and wherein comparing the oscillation wave to the oscillation signaturecomprises determining if the wave modulation frequency and the wavemodulation amplitude fall within the range of signature modulationfrequencies and the range of signature modulation amplitudes,respectively.
 4. The method of claim 2, wherein detecting from therotational speed an oscillation wave comprises decomposing therotational speed into a carrier wave and a modulation wave, the carrierwave corresponding to the rotational speed and the modulation wavecorresponding to the oscillation wave, and extracting the wavemodulation frequency and the wave modulation amplitude from theoscillation wave.
 5. The method of claim 1, wherein monitoring arotational speed of the shaft comprises determining the rotational speedusing a phonic wheel sensing assembly.
 6. The method of claim 1, furthercomprising confirming detection of the shaft shear event when theoscillation wave corresponds to the oscillation signature for apredetermined amount of time.
 7. The method of claim 1, furthercomprising monitoring a torque of the shaft and performing the methodonly when the torque meets a minimum threshold.
 8. A system fordetecting a shear of a rotating shaft of a gas engine, the shaftpositioned between a source and a load, the system comprising: a memorystoring a shaft oscillation signature determined as a function of knowncharacteristics of the shaft and associated with a shaft shear event;and at least one of: (a) at least one processor configured for executingprogram code; and (b) a circuit; the at least one of (a) and (b)configured for: monitoring a rotational speed of the shaft; detectingfrom the rotational speed an oscillation wave superimposed on therotational speed by detecting a first period below a lower threshold anda second period above an upper threshold, and detecting a rate ofoccurrence of the first period and the second period, the oscillationwave having a wave modulation frequency corresponding to the rate ofoccurrence and a wave modulation amplitude; comparing the oscillationsignature to the oscillation wave; detecting the shaft shear event whenthe oscillation wave corresponds to the oscillation signature; andissuing a shaft shear detection signal in response to detecting theshaft shear event, the shaft shear detection signal triggering a fuelshut-off of the gas engine.
 9. The system of claim 8, wherein theoscillation signature comprises a signature modulation frequency and asignature modulation amplitude, and wherein comparing the oscillationwave to the oscillation signature comprises comparing the signaturemodulation frequency to the wave modulation frequency and comparing thesignature modulation amplitude to the wave modulation amplitude.
 10. Thesystem of claim 9, wherein the oscillation signature comprises a rangeof signature modulation frequencies and a range of signature modulationamplitudes, and wherein comparing the oscillation wave to theoscillation signature comprises determining if the wave modulationfrequency and the wave modulation amplitude fall within the range ofsignature modulation frequencies and the range of signature modulationamplitudes, respectively.
 11. The system of claim 9, wherein detectingfrom the rotational speed an oscillation wave comprises decomposing therotational speed into a carrier wave and a modulation wave, the carrierwave corresponding to the rotational speed and the modulation wavecorresponding to the oscillation wave, and extracting the wavemodulation frequency and the wave modulation amplitude from theoscillation wave.
 12. The system of claim 8, further comprising a phonicwheel sensing assembly for sensing the rotational speed of the shaft.13. The system of claim 12, wherein the phonic wheel sensing assemblycomprises a phonic wheel having phonic teeth, and the phonic teeth arespaced such that a lowest sampled wave modulation frequency is at leastfive times a highest possible signature modulation frequency.
 14. Thesystem of claim 8, wherein the circuit is provided on a dedicatedcircuit board inside an aircraft electronic engine controller.
 15. Thesystem of claim 14, wherein the at least one processor communicates theshaft oscillation signature to the circuit for comparing with theoscillation wave.
 16. The system of claim 8, wherein the source is aturbine and the load is a fan, and the shaft is a lower pressure shafton a gas turbine engine.
 17. A system for detecting a shear of arotating shaft of a gas engine, the shaft positioned between a sourceand a load, the system comprising: a memory storing a shaft oscillationsignature determined as a function of known characteristics of the shaftand associated with a shaft shear event; a speed sensing device formonitoring a rotational speed of the shaft; at least one processorconfigured for executing program code for: detecting from the rotationalspeed an oscillation wave superimposed on the rotational speed bydetecting a first period below a lower threshold and a second periodabove an upper threshold, and detecting a rate of occurrence of thefirst period and the second period, the oscillation wave having a wavemodulation frequency corresponding to the rate of occurrence and a wavemodulation amplitude; comparing the oscillation signature to theoscillation wave; detecting the shaft shear event when the oscillationwave corresponds to the oscillation signature; and issuing a shaft sheardetection signal in response to detecting the shaft shear event, theshaft shear detection signal triggering a fuel shut-off of the gasengine.
 18. The system of claim 17, wherein the oscillation signaturecomprises a signature modulation frequency and a signature modulationamplitude, and wherein comparing the oscillation wave to the shaftoscillation signature comprises comparing the signature modulationfrequency to the wave modulation frequency and comparing the signaturemodulation amplitude to the wave modulation amplitude.
 19. The system ofclaim 18, wherein the oscillation signature comprises a range ofsignature modulation frequencies and a range of signature modulationamplitudes, and wherein comparing the oscillation wave to theoscillation signature comprises determining if the wave modulationfrequency and the wave modulation amplitude fall within the range ofsignature modulation frequencies and the range of signature modulationamplitudes, respectively.
 20. The system of claim 18, wherein detectingfrom the rotational speed an oscillation wave comprises decomposing therotational speed into a carrier wave and a modulation wave, the carrierwave corresponding to the rotational speed and the modulation wavecorresponding to the oscillation wave, and extracting the wavemodulation frequency and the wave modulation amplitude from theoscillation wave.